1. Field of the Invention
The present invention relates to a process for producing a glass mold. The glass mold obtained according to the process of the present invention can be preferably used for producing a molded glass article of high precision requiring no further post-operations (e.g. grinding or polishing) after molding.
2. Prior Art
Molds used for producing a molded glass article by press molding are required to cause no surface roughening due to the oxidation at high temperatures such as 300.degree.-700.degree. C. at which press molding is normally conducted, and to invite no fusion bonding of molded glass article to the mold after press molding. Said molds are further required to be processable at the inner surface to an optical mirror surface and to have mechanical strengths sufficient to withstand the impact applied at the time of press molding.
As materials for such glass molds, there have conventionally been used tungsten carbide (WC), a tungsten carbide (WC) - cobalt (Co) alloy and various cermets. These materials, however, cause surface roughening due to oxidation at high temperatures. As a material for a glass mold causing no surface roughening at high temperatures and processable at the inner surface into a mirror surface, there can be mentioned a SiC film obtained by a CVD method. However, this mold material, when made into a glass mold, invites fusion bonding of glass under press molding at high temperatures of 400.degree. C. or above.
Hence, in order to prevent fusion bonding of glass to a glass mold, a releasing film of carbon is formed on the surface of the mold. The releasing film of carbon includes various types such as (1) a diamond film, (2) a diamond-like carbon film, (3) a glassy carbon film and (4) a graphite film; however, each of these films has the following problems. The diamond film (1) has a high hardness and a relatively high adhesion to a .beta.-SiC film formed on a mold base; however, with respect to this diamond film, (a) the mold surface cannot be made into an optical mirror surface because it is impossible to obtain a diamond film consisting of very fine diamond particles of uniform size, and (b) the film removal necessary for the reclamation of mold cannot be effected by ashing according to the plasma oxidation method.
The diamond-like carbon film (2) is formed using a reactive gas consisting of CH.sub.4 and H.sub.2 as disclosed in Japanese Patent Application Kokai (Laid-Open) No. 281030/1986; accordingly, it is difficult to obtain this film in such thinness and smoothness as to enable the retention of the surface shape and surface roughness of mold base, thus making it necessary to polish the film surface to a mirror surface at each time of film formation. Further, the diamond-like carbon film as described in Japanese Patent Application Kokai (Laid-Open) No. 183134/1986, being a diamond film formed according to an ion beam sputtering method (this film is hereinafter referred to as "i-carbon film" in some cases), has at low temperatures a high hardness (e.g. Vickers hardness of 3000 or above) and a smooth surface; however, at press molding temperatures of, for example, 545.degree. C. or higher, the above film gets very brittle owing to its change from amorphous carbon to graphite and tends to peel off from the mold base; thus, the film is unsuitable as a releasing film of high temperature use. Moreover, since the ion beam sputtering apparatus is expensive, the above film formed with the apparatus is not desirable for use in the industrial production of a glass mold at a low cost.
The glassy carbon film (3) is oxidized easily and, owing to its structural weakness, is damaged easily, as described in Japanese Patent Application kokai (Laid-Open) No. 45613/1977 and U.S. Pat. No. 4098596.
The graphite film (4) is generally formed by thermal decomposition of CH.sub.4, acetone or alcohol. Therefore, the film contains hydrogen therein and, at press molding temperatures of 600.degree. C. or higher, gets soft and tends to peel off from the surface of the .beta.-SiC film formed on the mold base.
As described above in detail, the diamond film (1), the diamond-like carbon film (2), the glass carbon film (3) and the graphite film (4) each conventionally used as a releasing film of a glass mold for producing a molded glass article by press molding, have been unsatisfactory in at least one of such properties as processability of the mold surface into an optical mirror surface, glass releasability, hardness, resistance to oxidation at high temperatures and peeling resistance.